Referring to FIGS. 9A and 9B, press forming includes, for example, drawing and stretching. FIG. 9A is an example of drawing. A material metal sheet (referred to as blank) 100 is arranged into a die (upper die 20) from the periphery. FIG. 9B is an example of stretching. A draw bead 40 is provided so that a material metal sheet (blank) 100 is not arranged into the die (20) (Tekko Binran IV (Japanese), 3rd edition, pp. 252 and 259, edited by The Iron and Steel Institute of Japan). FIGS. 10A and 10B show a definition of a limit drawing ratio LDR described in the same document. Formability improves as the limit drawing ratio increases. In FIGS. 10A and 10B, reference humeral 10 denotes a punch which defines dies together with the upper die 20, and 30 denotes a blank holder.
As shown in FIG. 11A, hitherto, press forming has been typically performed by moving a punch 10 of an upper die 20 in a forming progress direction (in a direction in which a forming height increases) so that, for example, a material metal sheet 100 is formed by the upper die 20 located at an upper side in the figure and the punch 10 rising from a lower side in the figure until the shape of the material metal sheet (blank) 100 achieves a final target shape (the punch 10 reaches a top dead center). (Alternatively, a die 20 may be located at the lower side and a punch 10 may be located at the upper side. In this case, forming is completed when the punch 10 reaches a bottom dead center.) During the forming, in many cases, a blank holder 30 is arranged, and the forming is completed by moving the punch 10 while the metal sheet (blank 100) is held between the blank holder 30 and the upper die 20, to prevent a wrinkle from appearing at an outer edge of the blank 100.
A force of holding the metal sheet (blank 100) between the blank holder 30 and the upper die 20 is enough as long as the force prevents a wrinkle from appearing at the outer edge of the blank 100, and the force does not have to be excessively large. In the case of drawing in FIG. 9A, the metal sheet (blank 100) held between the blank holder 30 and the upper die 20 is drawn into a deep side of the upper die 20 while the metal sheet slides on the blank holder 30 and the upper die 20. Hence, if a blank holding force is excessively large, sliding may be inhibited, resulting in a crack likely appearing in the metal sheet (blank 100) during press forming. In the case of stretching in FIG. 9B, the draw bead 40 positively inhibits the metal sheet (blank 100) from sliding and prevents the metal sheet (blank 100) from being drawn into the deep side of the upper die 20.
Meanwhile, many types of forming defects may occur during press forming. In particular, when a part for press forming has a complicated shape or a material metal sheet (blank) have a high strength, a crack likely appears in the blank.
A typical method to prevent, the above problem may be, for example, correcting the shape of press forming dies (also simply referred to as dies) such as a punch and an upper die, changing the shape of a blank from its original shape, or changing the material of the blank to a special material.
However, applying the method of correcting the shape of the dies, or changing the shape or material of the blank requires a long time, a large amount of labor, and a high cost. Thus, a method of preventing a crack not relying upon the above method has been studied and developed.
Japanese Unexamined Patent Application Publication No. 2005-199318 discloses a method including, after a punch first contacts a metal sheet (blank) and forming is started, and before the punch reaches a stroke end and the forming is completed, detaching the punch from the metal sheet (blank), and resuming the forming of the metal sheet (blank) using the punch and an upper die.
Japanese Unexamined Patent Application Publication No. 2005-199319 discloses a method including, after a punch first contacts a metal sheet (blank) and forming is started, and before the punch reaches a stroke end and the forming is completed, detaching a blank holder from the metal sheet (blank), and resuming the forming of the metal sheet (blank) using the punch, an upper die, and the blank holder.
With the method of detaching the punch from the blank and resuming the forming of the metal sheet as disclosed in Japanese Unexamined Patent Application Publication No. 2005-199318, a lubricant flows again immediately after the punch is detached from the blank, and hence sliding performance is improved. This acts on improvement of formability. However, the action may be affected by the surface roughness of the dies or the type (kinematic viscosity) of the lubricant. The action may not be sufficiently obtained depending on the surface roughness of the dies and the kinematic viscosity of the lubricant to be used. An improvement has been desired.
The method of detaching the blank holder from the blank and resuming the forming of the metal sheet as disclosed in Japanese Unexamined Patent Application Publication No. 2005-199319 is in a similar situation. A lubricant flows again immediately after the blank holder is detached from the blank, and hence sliding performance is improved. This acts on improvement of formability. However, the action may be affected by the surface roughness of the dies or the type (kinematic viscosity) of the lubricant. The action may not be sufficiently obtained depending on the surface roughness of the dies and the kinematic viscosity of the lubricant to be used. An improvement has been desired.
It could therefore be helpful to provide a method capable of improving a forming limit at which a crack appears in a metal sheet and being easily applied to a large press machine for mass production with a low cost, without correcting the shape of dies, such as a punch and an upper die, or changing the shape or material of a blank to a special shape or material, even when the shape of a part for press forming has a complicated shape or a material metal sheet has a high strength.
It could also be helpful to provide a frame part for an automotive body manufactured by the press forming method and having excellent energy absorbability.